In a variety of environments, signals must be transmitted between diverse sources and circuitry using those signals, while maintaining electrical (i.e., galvanic) isolation between the sources and the using circuitry. Isolation may be needed, for example, between microcontrollers, on the one hand, and devices or transducers which use microcontroller output signals, on the other hand. Electrical isolation is intended, inter alia, to prevent extraneous transient signals, including common-mode transients, from inadvertently being processed as status or control information, or to protect the equipment from shock hazards or to permit the equipment on each side of an isolation barrier to be operated at a different supply voltage, among other known objectives and uses.
A variety of isolation techniques are known, including the use of optical isolators that convert input electrical signals to light levels or pulses generated by light emitting diodes, and then to receive and convert the light signals back into electrical signals. Isolators also exist which are based upon the use of Hall effect devices, magneto-resistive sensors, capacitive isolators and coil- or transformer-based isolators.
Optical isolators, which are probably the most prevalent, present certain well-known limitations. Among other limitations, they require significant space on a card or circuit board, they draw a large current, they do not operate well at high frequencies, and they are very inefficient. They also provide somewhat limited levels of isolation. To achieve greater isolation, optical-electronic isolators have been made with some attempts at providing an electrostatic shield between the optical transmitter and the optical receiver. However, a conductive shield which provides a significant degree of isolation is not sufficiently transparent for use in this application.
In the area of non-optical isolation amplifiers for use in digital signaling environments, U.S. Pat. No. 4,748,419 to Somerville, shows the differentiation of an input data signal to create a pair of differential signals that are each transmitted across high-voltage capacitors to create differentiated spike signals for the differential input pair. Circuitry on the other side of the capacitor barrier has a differential amplifier, a pair of converters for comparing the amplified signal against high and low thresholds, and a set/reset flip-flop to restore the spikes created by the capacitors into a logic signal. In such a capacitively-coupled device, however, during a common mode transient event, the capacitors couple high, common-mode energy into the receiving circuit. As the rate of voltage change increases in that common-mode event, the current injected into the receiver increases. This current potentially can damage the receiving circuit and can trigger a faulty detection. Such capacitively coupled circuitry thus couples signals that should be rejected. The patent also mentions, without elaboration, that a transformer with a short R/L time constant can provide an isolation barrier, but such a differential approach is nonetheless undesirable because any mismatch in the non-magnetic (i.e., capacitive) coupling of the windings would cause a common-mode signal to appear as a difference signal. Transformer cost and size may also be a negative factor, and transformers having cores of magnetic materials such as ferrites become inefficient at high frequencies and are not useful for coupling high-speed digital signals.
Commonly-owned U.S. Pat. No. 5,952,849 shows another logic isolator which avoids use of optical coupling. This logic isolator exhibits high transient immunity for isolating digital logic signals.
A need exists, however, for a less expensive, higher performance digital signal isolator with good dynamic characteristics at high frequencies or speeds.
Moreover, needs exist for logic isolators which provide improved low-cost bidirectional signal transmission capabilities and which can be configured for a variety of signal transmission configurations.
A need further exists for improved signaling schemes for use in isolators, to permit a logic isolator to be based on a single micro-transformer.
These needs are addressed with a logic signal isolator comprising, in a first aspect, a transformer having a primary winding and a secondary winding; a transmitter circuit which drives said primary winding in response to a received logic signal, such that in response to a first type of edge in the logic signal, a signal of a first predetermined type is supplied to the primary winding and in response to a second type of edge in the logic signal, a signal of a second predetermined type is supplied to said primary winding, the primary winding and the transmitter being referenced to a first ground; and the secondary winding being referenced to a second ground which is galvanically (i.e., electrically) isolated from the first ground and said secondary winding supplying to a receiver circuit signals received in correspondence to the signals provided to the primary winding, the receiver reconstructing the received logic signal from the received signals. The isolator's receiver may include circuitry for distinguishing between the received signals corresponding to the transmitted signals of the first type and second type and using the distinguished received signals to reconstitute the received logic signal. The signals of the first type may, for example, comprise multiple pulses in a predetermined pattern and the signals of the second type comprise one or more pulses in a different pattern. The signals of the first type also may comprise pulses of a first duration and the signals of the second type may comprise pulses of a second, distinguishable duration. At least one of the signals of the first or the second type also may comprise at least one burst. If both comprise bursts, they may be at different frequencies or be for different durations.
The transmitter circuit may be on a first substrate and the receiver may be on a second substrate electrically isolated from the first substrate.
The primary winding and the secondary winding desirably may be substantially planar windings arrange in a stacked arrangement with at least one of the windings substantially in or on one of the substrates. The primary winding then may be a bottom winding (closer to the substrate) and the secondary winding may be a top winding (further from the substrate). When the primary winding is a bottom winding, the isolator may further include a compensation network connected to the top winding for damping its response. Alternatively, the primary winding may be a top winding and the secondary winding may be a bottom winding.
According to another aspect of the invention, a bi-directional isolator is provided by including a second transmitter connected to drive said secondary winding in response to a second received logic signal, such that in response to a first type of edge in the second received logic signal, a signal of a third predetermined type is supplied to the secondary winding and in response to a second type of edge in the second received logic signal, a signal of a fourth predetermined type is supplied to said secondary winding, the secondary winding and the second transmitter being referenced to the second ground; and the primary winding being referenced to the first ground and said primary winding supplying to a second receiver circuit signals received in correspondence to the signals provided to the secondary winding, the second receiver reconstructing the second received logic signal. The isolator's second receiver may include circuitry for distinguishing between the signals received from the primary winding and using the distinguished received signals to reconstitute the second received logic signal. The signals from the first transmitter and the second transmitter may be similar or different.
According to another aspect, a digital logic isolator comprises a transformer having a primary winding referenced to a first ground and a secondary winding referenced to a second ground isolated from the first ground, means for providing to the primary winding signals of a first type in response to transitions of a first type in an input logic signal, means for providing to the primary winding signals of a second type different from the signals of the first type in response to transitions of a second type in the input logic signal, and means for receiving from the secondary winding signals corresponding to the signals of the first and second types and for reconstituting the input logic signal from them.
According to still anther aspect of the invention, a method of providing an isolated logic signal output in response to a logic signal input comprises providing a transformer having a primary winding referenced to a first ground and a secondary winding referenced to a second ground isolated from the first ground, providing to the primary winding signals of a first type in response to transitions of a first type in the input logic signal, providing to the primary winding signals of a second type different from the signals of the first type in response to transitions of a second type in the input logic signal, and receiving from the secondary winding signals corresponding to the signals of the first and second types and reconstituting the input logic signal from them.
The signals of a first type may comprise multiple pulses, the signals of the second type may comprise a single pulse and reconstituting the input logic signal may comprise distinguishing between the signals corresponding to said multiple pulses and the signals corresponding to said single pulses so as to provide an output signal reconstituting the transitions in the input logic signal. The signals of a first type or the signals of a second type comprise a burst. If both the signals of a first type and the signals of a second type comprise bursts, they may be distinguishable from each other by frequency, duration or other characteristic. A signal of the first type alternatively may comprise a pulse of a first duration and a signal of the second type may comprise a pulse of a second duration different from the first duration and distinguishable therefrom; and reconstituting the input logic signal may comprise distinguishing between received signals corresponding to the pulses of a first duration and the pulses of a second duration so as to provide an output signal reconstituting the transitions in the input logic signal.
According to another aspect of the invention, a logic isolator comprises a micro-transformer comprising, on a substrate, vertically stacked, a bottom winding and a top winding disposed over and insulated from the bottom winding, with a damping network connected across the top winding. A transmitter circuit receives a logic input signal and drives a signal to said bottom winding; and a receiving circuit is connected to receive from the top winding a signal corresponding to the signal driving the bottom winding and generates an output comprising a reconstituted logic input signal.
According to still another aspect, a logic isolator comprises a micro-transformer having, on a substrate, vertically stacked, a bottom winding and a top winding disposed over and insulated from the bottom winding; a damping network connected across the top winding; a transmitter circuit receiving a logic input signal and providing a transformer driving signal; a receiving circuit connected to receive from the transformer a signal corresponding to the driving signal and generating an output comprising a reconstituted logic input signal; and means for programming the isolator by coupling the driving signal to a selected one of the windings and coupling the receiving circuit to the other one of the windings. In such an isolator, the means for programming may comprise a fusible connection(s) programmed by blowing open a conductive path(s). The means for programming alternatively may comprise bond wires provided between the transformer windings on the one hand and the transmitter and receiving circuits, on the other hand. As a further alternative, the means for programming comprises programmable circuitry configurable to connect the transmitter circuit to either the top winding or the bottom winding and to connect the receiving circuit to the other winding. The programmable circuitry may include programmable switching circuits and a memory containing programming to control the switching circuits. The memory may be read-only memory.
According to yet another aspect, a logic isolator comprises a micro-transformer comprising, on a substrate, vertically stacked, a bottom winding and a top winding disposed over and insulated from the bottom winding; a damping network connected across the top winding; a first module coupled to the top winding capable of either receiving a logic input signal and providing a transformer driving signal or receiving from the transformer a signal corresponding to the driving signal and generating an output comprising a reconstituted logic input signal; a second module coupled to the bottom winding capable of either receiving a logic input signal and providing a transformer driving signal or receiving from the transformer a signal corresponding to the driving signal and generating an output comprising a reconstituted logic input signal; and means for programming the isolator such that the first module operates in the transformer drive mode while the second circuit operates in the receive mode or that the first module operates in the receive mode while the second module operates in the transformer drive mode. Various alternatives may be used as the means for programming. Such means may comprise, for example, at least one fusible connection programmed by blowing open at least one conductive path. As another example, the means for programming may comprise one or more bond wires provided between the transformer windings on the one hand and the first and second modules, on the other hand. The means for programming also may comprise programmable circuitry configurable to connect the first module to either the top winding or the bottom winding and to connect the second module to the other winding. Such programmable circuitry may include programmable switching circuits and a memory containing programming to control the switching circuits. The memory may include a read-only memory.
According to still another aspect, a dual-channel, bi-directional isolator comprises first and second micro-transformers arranged on a first substrate, each transformer having a top winding and a bottom winding. A first transmitter circuit is connected to drive the bottom winding of the first transformer; a second transmitter circuit is connected to drive the top winding of the second transformer. A first receiver circuit is connected to receive signals from the bottom winding of the second transformer. A second receiver circuit is connected to receive signals from the top winding of the first transformer. Preferably, but not necessarily, the first transmitter circuit and first receiver circuit are on the first substrate, and the second transmitter circuit and second receiver circuit are on a second substrate which is electrically isolated from the first substrate.
Yet another aspect of the invention is a single channel bi-directional isolator comprising a micro-transformer arranged on a first substrate, the transformer having a top winding and a bottom winding; a first transmitter circuit connected to drive the bottom winding; a second transmitter circuit connected to drive the top winding; a first receiver circuit connected to receive signals from the top winding; a second receiver circuit connected to receive signals from the bottom winding; and the first and second transmitter circuits transmitting so as to avoid interfering with each other. Preferably, but not necessarily, the first transmitter circuit and the second receiver circuit are on the first substrate and the second transmitter circuit and the first receiver circuit are on a second substrate which is electrically isolated from the first substrate.
According to still another aspect, there is provided a delay element for use in pulse generating circuits for generating pulses usable, for example, to drive a transformer as above-described. The delay element is useful for generating a delay interval, and therefore a pulse duration, of a length that is substantially independent of the supply voltage—i.e., is insensitive to variations in supply voltage. The delay element comprises first and second current sources which supply currents I1 and I2, respectively, in parallel, and a switching element. The sum of currents I1 and I2 is directly proportional to the supply voltage, and a threshold of the switching element is a predetermined portion of the supply voltage. The delay element may further include a capacitor of capacitance C, connected to a node in common with the input of the switching element and the current sources, chargeable by the current sources. Preferably, the first current source comprises a single transistor and a resistor, the resistor, of resistance value R, having one end connected to the supply voltage and the other end connected to said transistor. Current I1 =(VDD−VT)/R, where the transistor is a MOS transistor, VT is the threshold voltage of the MOS transistor, VDD is the supply voltage, I2=VT/R, I1+I2=VDD/R. The delay interval is then approximately 0.5RC if the switching threshold of the switching element is set to be VDD/2, and is relatively insensitive to changes in VDD. Such a delay element may be used in conventional pulse generating circuits that rely upon use of a delay element.